Metering apparatus and method for operating it

ABSTRACT

In a metering apparatus for the metered introduction of a first medium into an exhaust gas system of an internal combustion engine, which includes a conditioning apparatus in which also a second medium is introduced via a dispensing apparatus which is connected to the conditioning apparatus and via which the first medium and the second medium are introduced into the exhaust gas system, a supply unit for supplying a third medium, which is different from the second medium in its physical properties and/or its chemical composition, is connected to the conditioning apparatus for cleaning of the conditioning apparatus and the dispensing apparatus from constituent parts of the first medium by the third medium or a mixture of the second and the third medium.

This is a continuation-in-part application of pending internationalpatent application PCT/EP2007/001307 filed Feb. 15, 2007 and claimingthe priority of German patent application 10 2006 007 658.3 filed Feb.18, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to a metering apparatus for the metereddispensing of a first medium into an exhaust gas system of an internalcombustion engine, having a conditioning apparatus which can be fed thefirst medium and a second medium, and having a dispensing apparatuswhich is connected to the conditioning apparatus and via which the firstmedium and/or the second medium can be introduced into the exhaust gassystem, cleaning of the conditioning apparatus and/or the dispensingapparatus from constituent parts of the first medium being made possibleby the second medium. Furthermore, the invention relates to a method foroperating a metering apparatus.

DE 102 23 766 A1 discloses a method, in which a urea solution forexhaust gas aftertreatment is introduced as required via an injectionnozzle into an exhaust gas flow of a motor vehicle diesel engine. Inorder to clean the injection nozzle, a cleaning fluid is passed throughthe injection nozzle immediately after urea metering.

DE 101 50 518 C1 discloses a method for treating the exhaust gas of aninternal combustion engine, wherein a liquid reducing agent from asupply container is fed via a reducing agent line and a metering valveto a mixing chamber and is mixed there with a gas flow. The mixture isintroduced into an exhaust gas line via a mixture line. In order toprevent damage to the apparatus as a result of the reducing agentfreezing, the flow of the reducing agent is interrupted after theinternal combustion engine is switched off, and only gas is fed to themixing chamber. The mixing chamber, the mixture line, the metering valveand the reducing agent line are filled with gas via suitable valveswitching and are therefore protected against damage if the reducingagent freezes.

DE 102 54 981 A1 discloses an apparatus for exhaust gas aftertreatment,in which apparatus a reducing agent is stored in a supply container. Thereducing agent can be fed to an exhaust gas line via a reducing agentline. A switchable valve element, via which compressed air can beintroduced into the reducing agent line, is arranged in the reducingagent line. Here, the switchable valve element makes it possible forcompressed air to flow through the reducing agent line, optionally inthe direction of the exhaust gas line or in the direction of the supplycontainer. It is possible as a result to remove the reducing agent byblowing it out of the reducing agent line.

It is an object of the present invention to provide a metering apparatusof the type mentioned in the introduction, wherein however simple andeffective cleaning is made possible. It is a further object of thepresent invention to provide a method for operating a metering apparatusof this type in such a way that a satisfactory cleaning action can beachieved.

SUMMARY OF THE INVENTION

In a metering apparatus for the metered introduction of a first mediuminto an exhaust gas system of an internal combustion engine, whichincludes a conditioning apparatus in which also a second medium isintroduced via a dispensing apparatus which is connected to theconditioning apparatus and via which the first medium and the secondmedium are introduced into the exhaust gas system, a supply unit forsupplying a third medium, which is different from the second medium inits physical properties and/or its chemical composition, is connected tothe conditioning apparatus for cleaning of the conditioning apparatusand the dispensing apparatus from constituent parts of the first mediumby the third medium or a mixture of the second and the third medium.

Herein, the first medium preferably serves as working medium, by way ofwhich a desired physical and/or chemical effect can be achieved in theexhaust gas flow, for example temperature control of the exhaust gasflow, a direct chemical reaction with components of the exhaust gas or acatalytic action for exhaust gas aftertreatment. The second medium andthe third medium are two different media for effectively cleaning theconditioning apparatus and/or the dispensing apparatus at least fromconstituent parts of the first medium. Here, the apparatus can becleaned with the aid of the second medium, the third medium or anydesired combination of the first, second and/or third medium.Furthermore, a mixture formation comprising the first medium and one ofthe other media can be provided in the conditioning device. The meteringapparatus according to the invention makes it possible, in particular,to use in each case a particular medium or a mixture for cleaning theconditioning apparatus and/or the dispensing apparatus by which the bestpossible cleaning effect can be achieved under the given operatingconditions.

In one particular embodiment, a gas, in particular air, is provided assecond medium and a solvent, in which at least constituent parts of thefirst medium can be dissolved, is provided as third medium. Two mediaare therefore provided for cleaning the conditioning apparatus and/orthe dispensing apparatus, the cleaning action of which is based in eachcase on different working mechanisms. The feed of a gas makes itpossible, in particular, to remove the first medium in a simple way byblowing it out of the apparatus.

Any possible residues of the first medium which cannot be removed byblowing out, for example deposits or crystallizations, can be dissolvedand rinsed out by a feed of the solvent. The first medium can also beprovided as solvent.

In a further refinement of the invention, a metering element is arrangedin a line of the conditioning apparatus, and the feed of the thirdmedium is provided upstream of the metering element. A flow variable ofthe medium or mixture which flows through the line can be influencedwith the aid of the metering element, for example a throughput or a flowspeed. The metering element can be configured, in particular, as avalve, diaphragm valve, nozzle, diffuser or venturi nozzle. Meteringelements are particularly susceptible to contamination by the firstmedium. A flow as required of the third medium through the meteringelement makes it possible to effectively remove contamination, inparticular in the form of deposits and crystallizations.

In a further refinement of the invention, a sensor apparatus for ameasured variable is arranged in a line of the conditioning apparatus,and a feed of the third medium is activated if the measured variabledeviates from a reference value or a threshold value is reached.Variables which allow deductions to be made about a degree ofcontamination of the metering apparatus, for example a throughput or apressure difference, are preferably provided as measured variables.Corresponding values for the measured variable at identical operatingconditions and with an undisrupted throughflow preferably serve asreference values. The reference values are preferably defined as afunction of the operating point of the internal combustion engine or themetering apparatus. It is possible in this way to feed the third mediumas required if an operating state of the internal combustion engine orthe metering apparatus deviates from a reference state.

The method according to the invention for operating a metering apparatushaving a conditioning apparatus and a dispensing apparatus isdistinguished by the fact that the conditioning apparatus is fed a thirdmedium from time to time. Different media or mixtures are thereforeavailable for cleaning the conditioning apparatus and/or the dispensingapparatus from constituent parts of the first medium.

In one refinement of the invention, the third medium is fed into thesystem upstream of a metering element which is arranged in a line of theconditioning apparatus. It is possible in this way to clean the meteringelement if required with the aid of the third medium.

In a further refinement of the invention, a measured variable ismonitored in a line of the conditioning apparatus and/or the dispensingapparatus and the feed of the third medium is performed in the event ofa change with respect to a reference value by a predefinable amount orif a predefined threshold value is reached. Disruptions which arecaused, in particular, by contamination can be determined by monitoringa measured variable and comparing it with a reference value, so that thethird medium can be used as required.

In a further refinement of the invention, the second medium and thethird medium are introduced alternately within a predefinable timeperiod. This refinement of the method is suitable, in particular, forpreventative cleaning of the conditioning apparatus and/or thedispensing apparatus, before contamination is formed in a significantamount. Preventative cleaning of this type can take place, for example,under low load conditions or after the internal combustion engine isswitched off. An optimum cleaning effect can be achieved by thealternate introduction of the second medium and the third medium.

In a further refinement of the invention, a feed of the first medium isreduced during the feed of the second medium and/or the third medium. Inparticular, the feed of the first medium is suppressed completely here.Contamination by the first medium can be removed particularlyeffectively in the case of a reduced feed of the first medium.

The invention will become more readily apparent from the followingdescription of an exemplary embodiment thereof on the basis of theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE shows a block circuit diagram illustration of oneadvantageous embodiment of the metering apparatus according to theinvention.

DESCRIPTION OF A PARTICULAR EMBODIMENT

A metering apparatus 1 which is shown in the FIGURE serves to dispense aurea/water solution HWL into an exhaust gas system (not shown in greaterdetail) of an internal combustion engine. Here, the urea/water solutionis preferably introduced upstream of what is known as an SCR catalyticconverter for reducing nitrogen oxide. However, the metering apparatus 1is not restricted to this application, but rather can be used todispense any other desired liquid or gaseous media into an exhaust gassystem.

The metering apparatus 1 is part of a conditioning apparatus 2 whichcomprises a first feed line 4 for a first medium, a second feed line 6for a second medium and a third feed line 8 for a third medium. Here,the urea/water solution HWL is provided as first medium, air is providedas second medium and a solvent, in which, in particular, urea or ureacompounds are soluble, is provided as third medium.

The urea/water solution HWL is preferably stored in a supply containerwhich is not shown in the FIGURE. With the aid of a feed unit, theurea/water solution HWL can be removed from the supply container and fedto the conditioning apparatus 2 via the first feed line 4. Here, theurea/water solution HWL is preferably introduced under pressure. At itsoutlet-side end, the first feed line 4 is connected to a central line 10as will be explained in the following text. A first metering element 5,via which a throughput of the urea/water solution HWL can be set, isarranged in the first feed line 4. Here, the first metering element 5also advantageously makes it possible to shut off the first feed line 4.Furthermore, a spring-loaded first nonreturn valve 17 is arranged in thefirst feed line 4, which first nonreturn valve 17 reduces the risk ofmedia flowing back from the central line 10 into the first feed line 4.In a modified exemplary embodiment, the first nonreturn valve 17 in thefirst feed line 4 can also be omitted. This refinement makes it possibleto introduce one of the media air or solvent which are to be explainedin further detail in the following text or a combination of said mediainto the first feed line 4 from the central line in a targeted manner ifrequired. It is possible as a result to clean line sections of the firstfeed line 4 between the supply container of the urea/water solution HWLand the central line 10 from constituent parts of the urea/watersolution HWL.

Air can be sucked in from the surroundings and fed to the conditioningapparatus 2, preferably under pressure, via an intake apparatus which isnot shown in the FIGURE. The second feed line 6 is connected to theintake apparatus, it being possible for an air filter to be provided onthe inlet side to clean the surrounding air from any possiblecontamination. The second feed line 6 is connected at its outlet-sideend to the central line 10. A second metering element 7, by way of whichan air throughput can be set, is arranged in the second feed line 6.Here, the second metering element 7 also preferably makes it possible toshut off the second feed line 6. In an analogous manner to the firstfeed line 4, a spring-loaded second nonreturn valve 18 is arranged inthe second feed line 6, which second nonreturn valve 18 reduces the riskof media flowing back from the central line 10 into the second feed line6. In a modified exemplary embodiment, a compressed air store, in whichthe air which is to be fed to the conditioning apparatus 2 is stored, isprovided instead of the intake apparatus. This refinement isdistinguished by the fact that air which has already been conditioned,in particular cleaned, can be fed to the conditioning apparatus 2.

The solvent is preferably stored in a solvent supply container 15. Water(optionally mixed with surfactants) or a water/alcohol mixture can beprovided, for example, as solvent. The solvent can be removed from thesolvent supply container 15 with the aid of a feed unit in the form of apump 16 and can be fed to the conditioning apparatus 2 via the thirdfeed line 8. Here, the solvent is preferably fed under pressure. Thethird feed line 8 is connected at its outlet-side end to the centralline 10. A third metering element 9, via which a solvent throughput canbe set, is arranged in the third feed line 8. Here, the third meteringelement 9 can also preferably shut off the third feed line 8.Furthermore, a spring-loaded third nonreturn valve 19 is arranged in thethird feed line 8, which third nonreturn valve 19 reduces the risk ofmedia flowing back from the central line 10 into the third feed line 8.

The first feed line 4, the second feed line 6 and the third feed line 8are connected via a central line 10 to a dispensing apparatus 3 as willbe explained in the following text. A fourth metering element in theform of a venturi nozzle 12 is arranged in the central line 10 upstreamof a connecting point 13 of the first feed line 4. When a medium, inparticular air, flows through it, the venturi nozzle 12 brings about anincrease in the flow speed at its narrowest cross section. The venturinozzle 12 ensures that line parts upstream of the venturi nozzle 12remain largely free of constituent parts of the urea/water solution HWLwhen the urea/water solution HWL is introduced into the central line 10,through which air flows.

A sensor apparatus in the form of a pressure sensor 20 is arranged nextto the venturi nozzle 12. A pressure difference between the inlet-sideend and the outlet-side end of the venturi nozzle 12 can be measuredwith the aid of the pressure sensor 20. A pressure difference across theventuri nozzle 12 which is increased in comparison with a referencevalue can indicate contamination or blockage of the nozzle. The pressuresensor 20 is configured as a cylinder having a first chamber 21 and asecond chamber 22 (cf. FIGURE). Here, the first chamber 21 is connectedvia a first connecting pipe 23 to the central line 10 upstream of theventuri nozzle 12. The second chamber 22 is connected via a secondconnecting pipe 24 to the central line 10 downstream of the venturinozzle 12. The static pressure in the central line 10 upstream of theventuri nozzle 12 therefore prevails in the first chamber 21 and thestatic pressure in the central line 10 downstream of the venturi nozzle12 prevails in the second chamber 22. The first chamber 21 and thesecond chamber 22 are separated from one another by a spring-loadedplunger 25 or a diaphragm, with the result that a pressure differencebetween the two chambers brings about a deflection of the plunger 25 orthe diaphragm. The deflection can be converted via a signal transmitter(not shown in greater detail) into a preferably electric signal whichcan be transferred as an input variable via a control line 26 to acontrol unit 11 which is explained in the following text. In a modifiedexemplary embodiment, a further sensor apparatus for monitoring afurther measured variable is provided instead of the pressure sensor 20or in addition to it. The further sensor apparatus can be arranged inany desired feed line or in the central line of the conditioningapparatus. Variables are preferably monitored, with the aid of whichcontamination or blockage of the metering apparatus 1 can be determined;for example, a throughflow is detected continuously.

A dispensing apparatus 3 which is connected to the metering apparatus 1comprises a dispensing line 30 and an injection nozzle 31. The mediawhich are prepared in the conditioning apparatus 2 can be transported tothe injection nozzle 31 via the dispensing line 30. The injection nozzle31 is arranged within an exhaust gas line (not shown in greater detail)and makes it possible to dispense the prepared medium into an exhaustgas flow of the internal combustion engine.

The first metering element 5, the second metering element 7 and thethird metering element 9 can be actuated via a control unit 11. Thesignal from the pressure sensor 20 and further characteristic valuesfrom an engine control unit are preferably provided as input variablesin the control unit 11, for example operating duration, load state ortemperature of the internal combustion engine. A feed of the urea/watersolution HWL, the air and the solvent to the conditioning apparatus 2can be set with the aid of the control unit 11 as a function of anoperating state of the internal combustion engine, the exhaust gassystem and/or the metering apparatus.

Under normal operating conditions, a urea/water solution HWL ispreferably introduced as an aerosol-like mixture into the exhaust gassystem of the internal combustion engine. In order to produce a mixtureof this type, the first metering valve 5 and the second metering valve 7are opened, with the result that urea/water solution HWL and air are fedcontinuously to the conditioning apparatus 2. The urea/water solutionHWL is introduced into the air flow at the opening point 13 of the firstfeed line 4 into the central line 10, a finely distributed aerosol ofair and urea/water solution HWL being produced. It is ensured with theaid of the venturi nozzle 12 that the central line 10 upstream of theventuri nozzle 12 remains largely free of constituent parts of theurea/water solution HWL. Furthermore, the turbulences which are causedby the venturi nozzle 12 bring about finer atomization of the urea/watersolution HWL which is introduced downstream. The aerosol is guidedfurther in the dispensing line 30 to the injection nozzle 31 and isinjected into the exhaust gas system there.

When metering the urea/water solution HWL, there is the risk of ureacrystallizing out and the associated formation of deposits on the wallsof the lines, through which flow passes. Urea deposits lead to areduction in the cross section, through which flow passes, and thereforeto an increased flow resistance. Here, the venturi nozzle 12 isparticularly at risk because of the eddies which occur here and the lowstatic pressure in the flow. In the extreme case, the venturi nozzle 12can be blocked completely by urea deposits, with the result that the airflow in the central line 10 comes to a standstill.

In one preferred method for operating the metering apparatus 1, theconditioning apparatus 2 is therefore fed a solvent as required, inwhich urea can be dissolved. To this end, in a first method step, thepressure difference in the central line 10 between the inlet-side endand the outlet-side end of the venturi nozzle 12 is monitoredcontinuously with the aid of the pressure sensor 20. The measured valuesare transmitted via the control line 26 to the control unit 11, wherethey are compared with a reference value which is optionally a functionof the operating state. Reference values for the pressure difference canbe stored in the control unit 11, for example, in the form ofcharacteristic diagrams. A deviation of the measured values from thereference value by more than a predefined amount indicates contaminationof the venturi nozzle 12 by urea deposits. In this case, the feed of theurea/water solution HWL and the air via the first metering element 5 andthe second metering element 7 is interrupted in a following method step,and the third metering element 9 is opened. The urea deposits aredissolved in the solvent and rinsed out with the solvent flow. In afurther, following method step, the feed of the solvent is preferablyinterrupted again after a predefined time duration, and the feed of theurea/water solution HWL and the air is activated. If an increasedpressure difference across the venturi nozzle 12 is still determined viathe pressure sensor 20, the method can be repeated.

In a modified operating method, the second nonreturn valve 18 is used asa sensor apparatus, in order to monitor a throughflow of air through thesecond feed line 6. Urea deposits in the second feed line 6 or in thecentral line 10 bring about an increased flow resistance and therefore areduced throughflow of air. A deflection of a valve body of thespring-loaded second nonreturn valve 18 therefore also changes. Thedeflection of the valve body is converted via a signal transmitted (notshown in greater detail) into a preferably electric signal which istransmitted via a further control line 27 to the control unit 11. In thecontrol unit 11, the signal is compared with a reference value undernormal operating conditions. The feed of the solvent is activated in theevent of a deviation by more than a predefined amount or if a predefinedthreshold value is reached.

In an operating method which has been modified further, the firstnonreturn valve 17 can also be used as a sensor apparatus in ananalogous manner, in order to monitor the throughflow of the urea/watersolution HWL through the first feed line 4.

In a further, modified operating method, the first feed line 4 is alsocleaned from constituent parts of the urea/water solution HWL with theaid of the air, the solvent or a combination of said two media. Thisoperating method is suitable, in particular, for metering apparatuses,in which a return flow of media from the central line 10 into the firstfeed line 4 is made possible. In one method step, the feed of theurea/water solution HWL is interrupted, for example by a feed unit beingdeenergized. In a following method step, the first metering element 5 isopened, in order to cause pressure equalization between the first feedline 4 and the central line 10. Subsequently, in a following methodstep, the third metering element 9 is opened, in order to feed solventto the central line 10. The solvent is introduced from the central line10 into the first feed line 4 and, as a result, the first feed line 4 iscleaned from constituent parts of the urea/water solution HWL. Aftercleaning of the first feed line 4 with the aid of the solvent, the feedof the solvent via the third metering element 9 is interrupted again.Optionally, the second metering element can subsequently be opened andair can be introduced into the central line and, following this, intothe first feed line 4. This method makes it possible, in particular, toclean the first feed line 4 after the internal combustion engine isswitched off and optionally to fill it with air, as a result of which,for example, freezing of the urea/water solution HWL in the first feedline 4 at low temperatures is prevented.

In a further, modified operating method, the solvent is fed as afunction of an operating state of the internal combustion engine, forexample, after the internal combustion engine is switched off or in arelatively long low load phase. To this end, in a first method step, thefeed of the urea/water solution HWL via the first metering element 5 isinterrupted. In a following method step, the second metering element 7is opened and air is introduced into the conditioning apparatus 2. As aresult of the feed of air, the urea/water solution HWL is blown out ofthe central line 10 and the dispensing line 31. After a fixed timeinterval of, for example, thirty seconds, the air feed via the secondmetering element 7 is interrupted. In a further method step, the thirdmetering element 9 is opened and the solvent is introduced into theconditioning apparatus 2. Urea deposits which can have formed within theline parts, through which the aerosol flows during normal operation, arereleased by the solvent and rinsed out of the metering apparatus 1.After a further time interval of, for example, fifteen seconds, the feedof the solvent via the third metering element 9 is interrupted. For anoptimum cleaning action, the described method steps are advantageouslyrepeated multiple times. This method can particularly advantageously beused in a preventative manner, before relatively large urea depositshave formed in the lines.

If the metering apparatus 1 according to the invention is used in aninternal combustion engine of a motor vehicle, a cleaning agent from awindow washing system or a headlight washing system can also be used assolvent. To this end, it is advantageous to configure the third meteringelement 9 as a 3/3-way valve 9, as shown in the FIGURE. In a firstworking position of the 3/3-way valve, the solvent can be transportedfrom the solvent supply container 15 via the third feed line 8 to thecentral line 10. In a second working position, a feed of the solvent tothe conditioning apparatus is suppressed. In a third working position,the solvent can be introduced from the solvent supply container 15 intoa cleaning line 40 which in turn is connected to a window washing systemor headlight washing system (not shown in greater detail). Viacorresponding actuation of the 3/3-way valve by the control unit 11, thecleaning agent can be fed to the window washing system and/or themetering apparatus as required. Without departing from the scope of theinvention, further working positions of the third metering element 9 canalso be provided, in which, for example, the solvent can be introducedsimultaneously into the cleaning line 40 and into the third feed line 8.Likewise, intermediate positions can also be provided between thedescribed working positions. Furthermore, a further working position canbe provided, in which the third feed line 8 is connected to an outletline (not shown in the FIGURE) to the surroundings. In particular, thisworking position makes pressure dissipation possible of an excesspressure which possibly prevails in the third feed line 8.

In the described exemplary embodiment, the medium which is to be meteredis prepared in the conditioning apparatus 2 to form an aerosol and isfed to the exhaust gas system via the dispensing apparatus 3. However,it is likewise conceivable to feed the medium which is to be metered tothe exhaust gas system in pure form, that is to say without mixing itwith a further medium in the conditioning apparatus. A mixture formationcomprising the second and third media can likewise be provided in theconditioning apparatus 2.

1. A metering apparatus for the metered dispensing of a first mediuminto an exhaust gas system of an internal combustion engine, comprisinga conditioning apparatus (2) to which a first medium (HWL) and a secondmedium can be supplied, a dispensing apparatus (3) which is connected tothe conditioning apparatus (2) and via which at least one of the firstmedium (HWL) and the second medium can be introduced into the exhaustgas system for cleaning at least one of the conditioning apparatus (2)and the dispensing apparatus (3) from constituent parts of the firstmedium (HWL) by the second medium, and means for supplying a thirdmedium to the conditioning apparatus (2) for cleaning of theconditioning apparatus (2) and the dispensing apparatus (3) fromconstituent parts of the first medium by the third medium or mixtures ofthe second and the third medium the third medium differing from thesecond medium in its physical properties or its chemical composition. 2.The metering apparatus as claimed in claim 1, wherein the second mediumis a gas and the third medium is a solvent, in which at leastconstituent parts of the first medium (HWL) can be dissolved.
 3. Themetering apparatus as claimed in claim 1, wherein a metering element(12) is arranged in a main line (10) of the conditioning apparatus (2),and the feed of the third medium is provided upstream of the meteringelement (12).
 4. The metering apparatus as claimed in claim 3, wherein asensor apparatus (20) for sensing a measured variable is arranged in themain line (10) of the conditioning apparatus (2), and a feeder unit ofthe third medium is activated if the measured variable deviates from areference value or when a threshold value is reached.
 5. A method foroperating a metering apparatus for the metered dispensing of a firstmedium into an exhaust gas system of an internal combustion engine,comprising a conditioning apparatus (2) to which a first medium (HWL)and a second medium can be supplied, a dispensing apparatus (3) which isconnected to the conditioning apparatus (2) and via which at least oneof the first medium (HWL) and the second medium can be introduced intothe exhaust gas system for cleaning at least one of the conditioningapparatus (2) and the dispensing apparatus (3) from constituent parts ofthe first medium (HWL) by the second medium, and means for supplying athird medium to the conditioning apparatus (2) for cleaning of theconditioning apparatus (2) and the dispensing apparatus (3) fromconstituent parts of the first medium by the third medium or mixtures ofthe second and the third medium the third medium differing from thesecond medium in its physical properties or its chemical composition.said method comprising the step of feeding the third medium to theconditioning apparatus (2) intermittently from time to time.
 6. Themethod as claimed in claim 5, wherein a feed of the third medium isintroduced into the conditioning apparatus (2) upstream of a meteringelement (12) which is arranged in a main line (10) of the conditioningapparatus (2).
 7. The method as claimed in claim 6, wherein a measuredvariable is monitored in the main line (10) of the conditioningapparatus (2) or the dispensing apparatus (3) and the third medium isintroduced in the event of at least one of a change with respect to areference value by a predefinable amount and the reaching of apredefined threshold value.
 8. The method as claimed in claim 5, whereinthe second medium and the third medium are alternately introduce withina predetermined time period.
 9. The method as claimed in claim 5,wherein a feed of the first medium (HWL) is reduced when at least one ofthe second medium and the third medium is supplied to the meteringapparatus.